Original images created on flipcharts or whiteboards and other large format drawings are used for a variety of purposes. Typically, these images may be drawn by hand or are left in a form that makes them available solely in the format on which the images were created. For example, a hand drawn image on a flip chart sheet of paper is solely available on that sheet of paper. As a result, the image is generally not available to be shown to others in the absence of the sheet of paper itself.
There are several methods for making these types of images more readily available. For example, the image may be transcribed or manually written down or typed into a computer. Once in the computer, the transcribed image may be reproduced or transmitted at will. This method, however, is labor intensive and may result in inaccuracies due to errors in transcription.
Another method is to use an electronic whiteboard or a similar device in place of a flipchart. Such devices electronically sense the image on the whiteboard and provide a means for saving this image, for example, in a computer memory. There are drawbacks to electronic whiteboards including that they are limited in the manner in which images are placed on the whiteboard and are generally not portable.
It is also possible to use a photocopier to copy portions of a large image by manipulating the position of the original image to produce multiple copies corresponding to the different sections of the original image. This copying method is very cumbersome and may result in improper alignments of the original image reconstructed from the multiple copies.
Another alternative is to use a high-resolution digital camera to take a digital picture of the original image. These cameras, however, have a limited resolution capability.
Scanners for electronically forming an image of an original are also known. Typically, the captured image provided by a scanner is a pixel data array that is stored in memory in a digital format. A distortion-free image requires a faithful mapping of the original image to the pixel data array. Scanners typically include at least one means for imposing a mechanical constraint during the image capture process in order to maximize the likelihood of faithful mapping.
The four types of scanners known in the art are drum scanners, flatbed scanners, two-dimensional array scanners and hand scanners. Drum scanners attach the original to the surface of a cylindrical drum that rotates at a substantially fixed velocity. During the rotation of the drum, an image sensor is moved in a direction parallel to the rotational axis of the drum. The combination of the linear displacement of the image sensor and the rotation of the original on the drum allows the entire original to be scanned. At any moment during the imaging process, the current position within the pixel data array relative to the original can be determined by measuring the angular position of the drum and the translational position of the sensor. The position of the pixel data array with respect to the original is fixed as long as the original is properly attached to the drum, the drum rotation is properly controlled, and the sensor is properly controlled in its displacement along the linear path.
Flatbed scanners include a linear array sensor that is moved relative to the original along an axis that is perpendicular to the axis of the array. Thus, the position of the sensor in one dimension may be known by tracking the relative movement of the sensor. The position of the sensor in the perpendicular direction is implicitly fixed by addressing a particular array element at which intensity is to be measured. In one embodiment of the flatbed scanner, the original is placed on a transparent platen and the sensor, along with an image illumination source, is placed on a side of the platen opposite to the original. As long as the original is not moved relative to the platen, the pixel data array will be fixed with respect to the image to be captured. In another embodiment, the original is moved, rather than the sensor. This second embodiment is typical of facsimile machines. Precision paper transports provide a high degree of positional accuracy during the image-capture process.
Advantages of the drum and flatbed scanners include the ability to accommodate documents at least as large as A4, or 8.5".times.11" paper. Moreover, some of these scanners can handle A1 paper in a single setup. However, the scanners are not generally portable, since they require a host computer for control, data storage and image manipulation. The scanners are also generally incapable of accommodating documents of the size of a flip chart sheet of paper.
Two-dimensional array scanners may be used in the absence of mechanical encoding constraints, and require only that the array and the original be held motionless during an exposure period. A two-dimensional array of photosensitive elements directly accomplishes the mapping of the image of the original into a pixel data array. However, because a single 300 dpi mapping of an 8.5".times.11" original requires an image sensor having an array of 2500.times.3300 elements, i.e. 8.25 million pixels, these scanners are cost-prohibitive in most applications. For larger sized originals, the cost of these scanners is even more cost-prohibitive.
Conventional hand scanners require a user to move a linear array of electro-optical sensor elements over an original. The movement is by hand manipulation. Array-position information is determined using methods such as those employed in operation of a computer "mouse." As a linear sensor array is moved, the rotation of wheels, balls or rollers that are in contact with the original is sensed, and the position information is determined from the mechanical details of the rotation. In general, the surface of the mechanical element in contact with the original has a high coefficient of friction, e.g. rubber, so as to resist slip and skid. A cylindrical roller or two wheels connected by a rigid axle may be used to enforce a single translational degree of freedom during the scanning process. A straight-edge or other fixture is often used to fix the scan direction with respect to the original and to further enforce the translational constraint provided by the pair of wheels or the roller. Nevertheless, the position encoder approach is one that is often susceptible to slips and skips, so that the pixel data array loses its correspondence with the image on the original.
Hand scanners are typically connected directly to a personal computer for image data storage, processing, and use. Data rates from the image sensor tend to limit the scanning speed. The scanners provide feedback to the user, typically by means of green or red light emitting diodes, to maintain the appropriate speed for the desired image resolution. Some hand scanners use electromagnetic brakes to prevent the user from dragging the scanner over the image too rapidly, with the mechanical resistance increasing with increases in scanning speed.
Hand scanners utilize relatively small imaging arrays and generally cannot handle larger than A6 documents in a single pass. This requires stitching algorithms to join together multiple swaths of a larger document. Swath stitching is done in a separate operation by the personal computer. Scanning a multi-page business document or report with a band scanner is a tedious process that often yields low-quality results.
As previously noted, some type of fixture is typically used with a hand scanner. In the absence of a fixture, there is a tendency to impose some rotation as the hand scanner is moved across an original. If the user's elbow is resting on a flat surface during movement of the scanner, the rotation is likely to have a radius defined by the distance between the scanner and the user's elbow. As a consequence, the scanned electronic image will be distorted. Other curvilinear movements during a swath of the scanner will also create distortions.
To overcome this image distortion, a scanning device can be used that accommodates curvilinear movement during a scanning process, as described in U.S. Pat. No. 5,578,813 to Allen et al. entitled "Freehand Image Scanning Device Which Compensates for Non-Linear Movement," which is incorporated herein by reference. The scanning device disclosed in Allen et al. accommodates the curvilinear movement it and stitches together the multiple swaths in a low-cost manner and with a high degree of correspondence between the original image and the resulting image.
However, even using the scanning device of Allen et al. has its shortcomings with respect to large original images, such as on a flip chart or whiteboard. In particular, the image captured by this scanning device is large, both with respect to the amount of data stored that corresponds to the captured image, as well as the size of the image presented, both on screen and when printed out. For example, a 34".times.44" original document would be stored as a 34".times.44" document and printed out as a original document. Accordingly, what is needed is a scanning device that can store, display and print a large original image in more practical and useful sizes and formats.